Semiconductor lasers are key elements in the advancing performance of many systems, such as optical fiber communications, optical data storage, laser printing, optical clock generation and quantum key distribution. It is almost always the case that to achieve a high level of system performance, the emission characteristics of the laser must be maintained within a well-defined, and typically relatively narrow, window. In the most common and simplest cases, this window is defined to be an output power level window, and an optical photodetector is used to monitor and control the output power level from the device.
For edge emitting, semiconductor lasers, the power is typically monitored from the emission from the rear facet of the laser, while the useful power is extracted from the front facet. The placement of a photodetector at the rear of the laser places significant constraints on the geometry and dimensions of the laser package that the laser is mounted within.
For a typical vertical cavity, surface emitting laser (VCSEL) architecture, there is often only one useful optical output, and so there is a need for some mechanism for diverting part of the output beam to a photodetector so that the output may be monitored. In many cases, an optical beamsplitter is used to divert part of the output beam to the photodetector. Some edge emitting lasers also use a beamsplitter for diverting a small fraction of the main output beam to the monitoring photodiode.
The photodetector generates a laser power output signal that is relayed back to a control circuit that is external to the laser package. The control circuit then determines the appropriate DC current drive that should be supplied to the laser in order to maintain the output power at a constant target level. An important feature of this architecture is that one, and in some cases two, electrical leads are required for the sole function of transmitting the “laser power level output” signal from the laser package to the control circuit. These electrical leads are provided as pins protruding from the laser package. In general, for each additional internal measurement parameter that is transmitted to the control circuit, one or two additional electrical pins are required.
This multiplicity of electrical pins is wasteful of space. Furthermore, it serves to inhibit the number of signals generated for relaying to the control circuit. One of the primary drivers for the cost of a laser module is the size of the package, which is in turn driven, in part, by the number of independent electrical connections required to exit the laser package. Therefore, compromises are often made between keeping the cost of the laser package low and the number of control signals extracted from the package.